Partner: Tarmac

Grand Prix tracks around the world have to deal with the stresses and strains of Formula One cars. The engines can red-line, turning the tyres so fast that the vehicles can launch from a standing start up to 100kph in a little over two seconds. And then these incredible machines can brake from 200kph down to a total stop in a little under three seconds.

The forces the road has to survive means that it has to be made of special materials, and high-performance polymer binders have been used in grand prix tracks around the world.

When the operators of the Hochenheim track in Germany rebuilt the course in 2002, they chose to use Shell Bitumen’s Cariphalte binder, developed to strict racing conditions at a nearby facility. It has since been used at tracks including Sepang in Malaysia), Sakhir in Bahrain, Marina Bay in Singapore, and Yas Marina in Abu Dhabi. As well as at airports as far afield as Frankfurt and Singapore, Heathrow and Phnom Penh. But high-performance materials like this aren’t confined to racetracks and runways

We might not all recognise a concrete paved road when we see it, but we likely do when we hear it: that constant rumble due to the rough surface of the concrete, and the rhythmic sound of tyres crossing the joints between concrete slabs. Even on a road with a surface layer above the concrete, that rhythmic noise can be caused by cracks that are ‘reflected’ up, into the surface layer, from the joints in the paving layer below.

Stress absorbing membrane interlayers

For this article we looked at how high-performance binders are being shown to be a sustainable way to prevent cracking in paved roads. The binders are used in a ‘SAMI’ or stress absorbing membrane interlayer. This SAMI is laid in between the lower layers of the road and its running surface, its strength and flexibility allowing it absorb stresses from movements in the lower layer, and prevent them passing to the surface.

In 2015, Tarmac collaborated with Nottingham University to better understand cracking on concrete paved roads. They agreed it was a significant issue and made improvements to the design of materials used in products – Tarmac’s offering is called SAMI Ultilayer. They also created a crack development model to predict the performance of different solutions. One key feature of the Stress Absorbing Membrane Interlayer is undoubtedly the ability of the binder to absorb movement.

With this in mind, Tarmac turned to Shell and their expertise with binders, as Shell Bitumen’s Connor Campbell explains, “Shell initially piloted what we call the crack relief layer, which is essentially a SAMI, in China. And it was developed under the trademark Strada. It was a patent that we held for a number of years.”

Since 2006 they have laid 2000km of highways using a SAMI, with a Cariphalte dense mixture and reported very good performances. Cariphalte is Shell Bitumen’s polymer-modified high-performance bitumen and Cariphalte isn’t just used in SAMI roads. It is used more generally wherever a road might be subject to conditions that call for greater strength or flexibility.

“It’s designed for more close-graded or dense asphalt applications, which SAMI falls under,” says Campbell. “It’s a very low void content, dense asphalt type and the polymer modified bitumen that we designed fits well with that type of aggregate grading and asphalt design.”

Types of road

In the UK, products like this are one solution to the problem of cracks in the surface layer of concrete paved roads, and roads in the UK have been laid in three main ways, says Tarmac’s David Markham.

“There’s fully-flexible, which are all asphalt pavements, which don’t self-crack if you like. Then we’ve got flexible composite pavements where part of the structural element, the structural base is a cement-bound material, which you pre-crack to control subsequent reflective cracking.

“And then we’ve got what I call proper concrete pavements, which are jointed, in, in most cases, but not always, jointed concrete. So at the time of construction, there is a joint in there. And those joints are, I guess, partly for construction purposes, but they’re also to accommodate thermal movement that you get.”

Historically, concrete paving was used in the UK where there was a ready supply of suitable materials. In many ways, it is a perfectly sensible approach, reducing movements of materials around the country.

“You can drive perfectly safely on concrete pavements, but people tend not to like them because they are quite noisy,” says Markham. “You’ll certainly know if you’ve got a concrete road round you. And the people like to overlay them with asphalt to particularly to control noise, because we can very significantly reduce the noise on jointed concrete pavement by overlaying it with asphalt.”

And concrete paved roads, surfaced with asphalt, are prone to a problem called reflective cracking

“These concrete slabs expand and contract as the temperature goes up and down, and although the movements are small, they pull the pavement apart and then back together as the day thermally cycles. And it’s that movement that partly contributes to any asphalt that’s over the top of them being stressed and eventually cracking.”

When heavy vehicles travel on these roads, yet more stress is added. If there is a pronounced joint in the concrete slab, as a heavy goods vehicle drives over it, there is a discontinuity of vertical load, and the joint will move.

A noisy, cracked road is not pleasant to drive on, but the problems caused by cracks can run deeper.

“It’ll let water in, and water in any sort of construction material, including highway pavements, is never a good idea,” says Markham. “We spent a lot of time learning that the hard way in the UK, and then doing as much as we could to prevent water from entering all different types of pavements. You’ll start to get fretting eventually, so you’ll never get a perfectly clean crack. And the asphalt will start to unravel slowly from that point. So once that crack forms, you’re on a sort of slope down to eventual failure.”

Once a concrete paved road is showing cracks in its surface layer, highway owners should consider repairs. The challenge is to find a way to do so that reduces materials used, and the cost of installation. That was the case for Norfolk County Council, more than a decade ago, when it became clear that cracks were forming on the surface of the A140, near Diss. The council decided to work with its testing facility at Norse Group to assess different approaches to repairing roads like this.

Getting inside the problem

Simon Shearwood is a senior engineer, focussing on pavements and surfaces, at the laboratory, which is an independent body, wholly owned by the council.

“On the A140 near diss on the Norfolk Suffolk border we have a dual carriageway, which is three and a half kilometres long. The dual carriageway is continuously reinforced concrete pavement. The pavement was deteriorating quite quickly, sometimes overnight. Large punch outs were happening. So we were having a lot of problems with this pavement,” says Shearwood.

The Norfolk team considered three approaches: completely replacing the asphalt and the pavement below; using a material called Geogrid; and using a SAMI layer.

“We looked at some solutions at the time one was basically to take up the existing concrete road and start from scratch. We looked at using grids, and also we looked at the Sami which was the offer from Tarmac at the time. if we were to take the concrete road up, we couldn’t control the costs. We had a limited budget. And when we started looking at how we could take the road up and start again, the actual breaking up the concrete the removal of the reinforcement, within the concrete etc. The cost became uncontrollable.”

A budget-conscious local authority couldn’t accept the risk of costs spiralling as contractors dug up the old road, including the concrete paving below. So how about using Geogrid? That’s a layer of manufactured material, which is laid on top of the concrete paving from a roll, somewhat like wallpaper. It requires different equipment, and different contractors, to a standard asphalt road.

“We looked at grids as well. But to use grids would mean a third party coming in as well. So if the asphalt supplier was there, they would have to stand while the grids were laid. And then the external supply could lay over the grid. So there were delays,” says Shearwood.

And the challenges Geogrid approaches pose don’t end at installation. They can come back years or decades later, when the geo grid road needs to be replaced, says Tarmac’s Markham.

“As an asphalt producer, we don’t like anything coming back in for us to recycle that isn’t just pure asphalt. We don’t like bits of fibre and detector loops and things that can come up from a road that aren’t basically stone and bitumen, because that’s what we have to supply back out as, as fresh asphalt is stone and bitumen,” says Markham.

The ideal solution then is one which preserves as much of the road as possible, that can be laid using the same equipment as the rest of the road surface, and can later be recovered and reused, without any materials being planed up along with the stone and bitumen.

The initial benefit Shearwood saw in using Tarmac’s SAMI Ultilayer was that it required no additional equipment or contractors.

“Tarmac would lay it with an asphalt paver, so the SAMI would be laid directly onto the concrete road with an asphalt paver,” says Shearwood.

On this project, as on the Chinese highways, the surface layer was laid relatively thick, and high performance polymer binders were also used in these upper layers.

“We have a 25mm SAMI, then above that we have nearly 175mm or more of asphalt on top of that. So as a whole life costs in there, the pavement itself you know, to get back to the Sammy and down to concrete, we’d have to take all that off again, if we want to start from scratch,” says Shearwood. “So there’s a lot of material there and a lot of expensive material there. So the long term view is that SAMI is there for life, it’s working for a long, long while, the only treatment we would have to do is for the surface course.”

Recently, Shearwood returned to the site and inspected the road and it still looks brand new.

“When we walked up and down it. It didn’t show signs of cracking after 12 years. That’s quite good. No, that is very good. We’ve invested a lot of money and we’ve got we’re getting return. But sometimes you have to invest the money upfront to get the longevity and the return, and we’re getting that return at the moment.”

On this project, and on those in China, the aim was to extend the lifetime of roads as long as possible. By using SAMI and a thick surface layer, with its own high performance modified binders, it is possible to resurface a concrete paved road, or build a new highway network, that will last for decades, with only minimal resurfacing work needed. But part of the beauty of SAMIs is that they can be used to protect a relatively thin surface layer from reflective cracking.

Making and using the material

The SAMI layer, of course, isn’t constructed purely out of binder. The selection of materials used by an asphalt producer like Tarmac is also key to the layer’s ability to absorb stress, without needing to be laid thick.

Markham adds, “It’s a generally a washed crushed rock fines, which is material so that’s a sort of 4mm, downy dust from a hard stone. We generally wash it because that’s what gives you the best control over the final product and allows us to add limestone filler and then a high proportion of a high performance polymer modified binder. SAMI is designed to be laid quite thinly, which is why we use a dust material, a sort of 4 or 5mm aggregate size material.”

The science of cracking is complicated. As Markham points out, it’s the stuff of PhD theses, but a general pattern can be seen.

“The highest stress point is right next to the concrete. And that’s where you want a really flexible material that can accommodate the movement that’s been forced onto the asphalt by the concrete that’s underneath.”

As the SAMI can be laid in a thin layer, flexing with the concrete below, and absorbing stresses without them reflecting up into the surface, it can reduce the overall thickness of the road, this makes it ideal for resurfacing old roads, and preventing cracks in future.

“Where you get the real benefit of these materials,” says Markham, “is if you want to overlay something, but the thickness that you can use is constrained. Now that might be because, for example of structures, so if you’re going underneath a whole series of bridges, you can’t lay a thick layer of asphalt up your job, because that would remove the headroom underneath the structures for heavy goods vehicles. And similarly, if you’ve got concrete barrier, down the centre of a road or your drainage is all set up, you can’t necessarily put a thick layer of asphalt on top.”

Campbell continues, “SAMIs are generally laid quite thin. The one that we’ve developed, and the one that Tarmac use, is generally laid at 2.5cm over concrete or over a difficult sub base to work with, so it is very, very thin with quite a high bitumen or binder content compared to conventional roads. So most typical asphalt designs will have four or five maybe 6% bitumen, but we’ve got a lot more almost double in a SAMI.

On some roads, using a SAMI layer allows highway owners to reduce future resurfacing work without interfering with existing road features, but it also has clear environmental benefits. As Markham explained, using standard asphalt materials, rather than a geogrid, allows for road surfaces to be easily recovered and reused. Reuse is only one element of sustainability, equally important is reducing the use of natural resources, and SAMI contributes to that.

“You can put down 25mm of Sami and then just a surface course on top. And when we’ve done that there are examples that we’re continuing to monitor on the network that have been down for on relatively heavily traffic roads for a very long time. Now they’re really performing well,” says Markham.

But making the most of a material such as this isn’t a routine process.

“the design is much more sensitive than a conventional asphalt mix the volumetrics are very importan,” says Campbell. “So choosing the right modified binder and getting the right type of aggregate, the right gradient of aggregate and the right mix of the two is key to get enough performance.”

It takes careful testing of the material’s properties, and close collaboration between the supplier and its customers to determine the best mix and thickness of materials.

In Norfolk, there are few, if any, concrete paved roads left in need of treatment, but the insights from Norse Group’s testing can be drawn on by road owners across the country.

“We have less concrete roads than some authorities,” says Shearwood. “Places like Suffolk and Essex have a lot more concrete roads, you know, the home counties around London, even some London boroughs have a lot of concrete roads.”

Indeed, Tarmac has been working closely with the UK’s National Highways to assess the viability of using SAMI across the network’s concrete paved roads.

“There’s one that we’ve continued to monitor with national highways. It’s on there, it’s on the strategic road network, where as part of a larger jointed concrete overlay, which had come to the end of its life, it needed treatment,” says Markham. “they had the foresight to include in that a number of trial panels. So as well as the sort of conventional solution, they had some low lower grade treatments, which were meant to crack early. As well as Geogrid sections and a section that was essentially Ultilayer SAMI. And they’ve continued to monitor them for performance to see when they start to crack, and how much cracking there is, in each of those test panels.”

For Markham it was a good result because the conventional solutions cracked first, the SAMI panels had yet to crack, when he last visited.

This article is based on Engineering Matters episode #162 Smooth as Glass: The Road Ahead, click here to listen